Currently, the main purpose of isolated synchronous rectification is to replace secondary side Schottky rectifier for low output voltage and large output current applications to achieve higher efficiency, smaller foot print, and lower system cost. Low conduction voltage drop Schottky rectifier has a forward conduction voltage drop at about 0.2V to 0.3V while synchronous rectifier has a conduction voltage drop about less than 0.15V under the circumstance of reasonable design. Such synchronous rectifier may substantially reduce the rectification power loss and improve system efficiency.
FIG. 1 illustrates a schematic diagram of a conventional synchronous rectification control device. The synchronous rectification control device includes a transformer 101, a synchronous rectifier 102, an output capacitor 103, a VCC power supply resistor 104, a VCC bypass capacitor 105, a time constant setting resistor 108, another time constant setting resistor 107, and a synchronous rectification control chip 106. Because the resistor network jointly formed by the time constant setting resistor 108 and the time constant setting resistor 107 makes the primary side turn-on decision, the synchronous rectification control chip 106 determines the turn-on of the primary side power transistor by integrating the signals of DET pin and AE pin. The VCC pin supplies power to the chip.
However, the control mechanism described above is complex and requires many components. The power supply by the VCC pin comes from the output voltage Vout. When the entire control system operates in a low output voltage mode, the VCC may not supply sufficient power, causing the malfunction of the synchronous rectification control chip 106. The rectification function may only be supported by a parasitic body diode of the rectifier, causing excessive heat and loss of efficiency.
FIG. 2 illustrates a schematic diagram of another conventional synchronous rectification control device. The synchronous rectification control device includes a transformer 201, an output capacitor 202, a VCC bypass capacitor 203, a synchronous rectification driving chip 204, and a filtering capacitor 205. This type of control structure totally depends on voltage determination to control the turn-on and turn-off of the synchronous rectifier. The lack of primary side turn-on recognition mechanism may easily cause erroneous operation of the synchronous rectifier and potential device explosion symptom.
Therefore, the conventional synchronous rectification control circuits are unable to operate properly under the circumstances of complex system, lacking dual voltage determination mechanism, and inefficient.